There is a difference between disagreeing with someone's estimation of costs versus benefits, and finding their reasoning "perplexing." In fact I think the benefits of criminal drug enforcement does not outweigh the cost of enforcement. But I can easily see how someone might come to the opposite conclusion. Crack babies aren't a made-up problem. Many homeless are drug users. Meth really is ripping apart families in rural America. All you have to do to conclude we should make those substances illegal is underestimate the massive cost of drug enforcement. And that's not an irrational thing to do--its not like either side has concrete numbers to point to.